This invention relates generally to a circuit for producing high voltage pulses and more specifically to an apparatus for producing high voltage pulses from a voltage multiplier circuit driven by a tuned flyback transformer circuit.
It is well known that oil and gas are more likely to be discovered in commercial quantities from formations of a relatively porous nature than from those formations which are less permeable. It is also well known in the industry that oil or gas producing formations may be located by passing a pulsed neutron source through a borehole and measuring secondary gamma ray radiation developed from the neutron irradiation of the formations surrounding the borehole. One such method of neutron-gamma ray logging is referred to as carbon-oxygen logging.
In carbon-oxygen logging of this type, a pulsed neutron source is pulsed at a very high repetition rate and the inelastic gamma ray energy spectrum, resulting from the formation bombardment by fast neutrons, is analyzed. In the past there has been available two pulsed power supply systems for providing the extremely high potentials required to cause the D-T reaction used to generate these fast neutrons. One such power method uses a belt-type electrostatic generator, such as a Van de Graaff generator, while the second power supply system utilizes a SCR pulsing circuit combined with a voltage multiplying circuit to produce the potentials desired.
In a belt-type electrostatic generator, as a Van de Graaff generator, an electrically insulated belt is driven by a pair of spaced pulleys. Electrical charges of one polarity are applied to the belt adjacent one pulley. Adjacent the other pulley, the charges are removed from the belt at a point within a high voltage terminal where they are accumulated to develop and maintain the required high voltage. Many problems have been encountered with such generators, particularly as used within subsurface instruments, such as those employed in nuclear well logging. Electromechanical generators of the Van de Graaff type suffer from excessive motor noise and sparking which can affect the output of the ion source. Further, these generators do not provide a constant output and are limited in the physical and thermal abuses to which they can withstand. These difficulties have severely limited the efficient use of such generators in well logging.
As stated above a second pulsed power supply system used in radioactivity logging utilizes a SCR pulsing circuit to feed a voltage multiplier string. In this system, as described in U.S. Pat. No. 3,714,468, issued Jan. 30, 1973 to E. C. Hopkinson, and assigned to the assignee of the present application, a unijunction oscillator triggers a SCR thereby producing a voltage pulse. The voltage pulse is transformer coupled to a voltage multiplier circuit which is connected to the ion source. While a supply of this type functions well at relative low frequencies, as example 1KH it will not operate at the higher pulse rated required for spectrum analysis using the so-called carbon logging instrument. Further, this power supply system requires an additional negative voltage source for providing the negative potential need on the grid of the ion source.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing circuitry for generating high voltage pulses at high repetition rates to be used in powering an ion source, and further providing the negative grid voltage potential from the same supply and a tracking signal for synchronizing the detector circuitry of the radioactivity instrument.